niteria authored Sep 13, 2017 and Ben Gamari committed Sep 13, 2017

This sprintf is safe thanks to the guarantees on the format strings that
we pass to it.  Well, almost. The GR_FILENAME_FMT_GUM format would not
have satisfied them if it was still used.

If someone makes a mistake that's a potential privilege escalation,
so I think it's reasonable to switch to snprintf to protect against
that remote possibility.

Test Plan: it builds, CI

Reviewers: simonmar, bgamari, austin, erikd

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3944